Cellphone users nationwide just received a ‘Presidential Alert.’ Here’s what to know.

Beginning at 2:18 p.m. Eastern time Wednesday, Americans across the country were interrupted by an urgent notification on their cellphones.

“Presidential Alert,” the message read. “THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed.”

Blasted out by cell towers nationwide over a 30-minute period, the message was expected to reach some 225 million people in an unprecedented federal exercise.

Wednesday’s test of the Wireless Emergency Alerts system is the first time that emergency management officials have used the nationwide alerting capabilities reserved for the office of the president.

Reactions to the alert were swift. #PresidentialAlert became a trending topic on Twitter, where some said the message interrupted their matinee movie showings. Others reported getting the notifications pushed to their smartwatches, as still others took the opportunity to crack jokes. Some said they had received the notification repeatedly, some as often as three to nine times.

Came through, buzzed like mad and also pushed to my Apple Watch. Was kind of annoying. I survived.

FEMA and Apple didn’t immediately respond to a request for comment on the matter. AT&T said it saw “no issues” with its alert, and that it sent it out when it received the notification from FEMA.

Many cellphone users may be familiar with alerts they receive as warnings about flash flooding or missing children. But those emergency weather alerts and AMBER alerts can only be sent on a regional or statewide basis, not nationally and all at once.

The presidential alert is designed to let the White House inform the entire country almost instantly of grave public emergencies, such as a terrorist attack or an invasion, according to the Federal Emergency Management Agency. And while Americans can choose not to receive weather and AMBER alerts, they cannot opt out of presidential ones.

Not everyone was pleased about the decision to send out the test alert. Minutes before the alert was sent out, a federal judge in New York rejected a lawsuit by three people who claimed that the presidential alert violated the First and Fourth Amendments, because Americans could not opt out. The judge explained her decision by saying the claims were too speculative, according to the Associated Press, and that it was motivated by concerns that President Trump sought to turn the wireless emergency alert system into “a second Twitter feed.”

Despite its name, the presidential alert was not issued by President Trump directly. But in general, the process calls for the president — or his representative — to authorize FEMA to send an alert on the White House’s behalf.

📳 TODAY (10/3): At 2:18 PM EDT, we will be testing the Wireless Emergency Alert system nationwide. Expect a message box on your phone along with a loud tone and vibration.

Before a presidential alert shows up on cellphones, it must be routed to wireless carriers, such as AT&T and Verizon, through an online system called the Integrated Public Alert and Warning System, or IPAWS.

The cellphone alert is also accompanied by a similar test of the Emergency Alert System, an older system that handles radio and television.

The federal government is required by law to test its alerting systems every three years. FEMA last tested the EAS in 2016 and 2017, and the first wireless emergency alerts were sent in 2012.

Not all cellphones may have received Wednesday’s alert, however. If you weren’t in range of a cell tower or if your phone was switched off, you probably missed the message. That was expected to be the case for only about a quarter of the nation’s cellphones.

That three out of four phones probably received the warning is a reflection of our always-on, technologically connected times. Yet the same technology that has helped connect millions of people also has the potential to cause chaos at Internet speeds.

In January, America’s emergency management technologies made headlines when a state-level official mistakenly sent an alert to Hawaii residents warning them of an inbound ballistic missile attack. The report was false, but it sent Hawaiians scrambling for cover, seeking shelter in gymnasiums, mountain bunkers and even in manholes.

A subsequent investigation by the Federal Communications Commission found that the staffer responsible for the message had thought an attack was truly underway, having misinterpreted a drill.

FEMA officials have said the Hawaii incident played no role in the scheduling of Wednesday’s test.

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Brian FungBrian Fung covers business and technology for The Washington Post. Before joining The Post, he was the technology correspondent for National Journal and an associate editor at the Atlantic. Follow